The present invention pertains to the hydrogen peroxide bleaching treatment of chemical pulps.
The unbleached chemical pulps or cellulose pulps obtained by digesting cellulose-containing materials according to the so-called sulfite or sulfate or Kraft process with soda or carbonate need to be bleached so that the products obtained may have the quality required for their intended use.
The lignin content in the pulp, expressed by the kappa number in the trade, practically determines the global quantity of the chemical bleaching agent to be used.
The lower the initial kappa number, the more efficiently and economically the bleaching can be achieved in a limited number of stages.
The above-mentioned digestion processes unfortunately do not permit a low kappa number to be reached directly without marked deterioration of the mechanical properties of the cellulose fibers.
The application of chlorine and its oxidizing derivatives also fails to permit the direct reduction of the kappa number of the resinous wood chemical pulps to values close to 10 without the formation of intensely colored, polluting and corrosive effluents.
Also, in the case of resinous wood, hydrogen peroxide, an oxidizing agent which can be used most easily, only leads to kappa numbers between ca. 20 and 25 if applied directly to the chemical pulps in an alkaline medium.
A pretreatment in an acidic medium as was described, for example, in Japanese Patent Specification No. 1981-[Sho 76]102,103 or in French patent application published as No. 2,520,397 reduces this value to 15-20 but creates the above-mentioned difficulties due to the effluents which result from the alternation of the stages carried out in an acidic bath with stages carried out in an alkaline bath, which is implied by the method proposed.